1. Field of the Invention
The present invention relates to a printing apparatus and a method of processing printing data.
2. Description of the Related Art
A printing apparatus includes a printhead in which a plurality of nozzles (ink orifices) are arranged, and performs printing by alternately performing an operation of printing a dot by each nozzle while scanning the printhead on a printing medium, and an operation of conveying the printing medium. Dot printing is performed by driving printing elements formed in one-to-one correspondence with the nozzles. Note that the scanning direction of the printhead is called a “main scanning direction”, the conveyance direction of the printing medium is called a “sub scanning direction”, and the two directions intersect each other.
FIG. 31 is a schematic view exemplarily showing dots d printed on a printing medium P when printing is performed by a time-divisional driving method by using a printhead H. Note that the time-divisional driving method is a driving method of performing printing by dividing a plurality of printing elements corresponding to a plurality of nozzles nz into, for example, N groups G (G0 to GN-1), and sequentially driving the printing elements of each group G one by one. Note that printing elements to be driven at the same time in the groups G0 to GN-1 are classified as a “block”, and 16 blocks B (B#0 to B#15) exist in the example shown in FIG. 31.
As shown in FIG. 31, an image based on printing data input to the printing apparatus is formed by a plurality of dots d printed in each column on the printing medium P. More specifically, the printing data input to the printing apparatus is first developed (expanded) so that a plurality of printing elements can be driven in a predetermined order. After that, each printing element is driven based on the developed printing data, and the nozzle nz corresponding to the printing element discharges ink (a printing agent), thereby printing the dot d for forming an image in a corresponding column on the printing medium P.
If the direction in which the plurality of nozzles nz are arranged has a tilt with respect to the sub scanning direction, this tilt produces a shift of a printing position (a shift from the intended position of the dot d). That is, the dot d to be printed in a given column is printed in a position (for example, an adjacent column) deviated from the given column due to the tilt. This will be explained with reference to FIG. 32.
FIG. 32 is a schematic view showing, in the same manner as in FIG. 31, a printhead H having a nozzle array whose arrangement direction has a tilt with respect to the sub scanning direction, and dots d printed when printing is performed by using the printhead H. For example, of the dots printed by nozzles nz of a group G2, two dots on the downstream side in the conveyance direction of a printing medium P are intended to be printed in column 0 but are printed on the left side of column 0 due to the tilt of the nozzle array. A shift of the printing position like this may decrease the image quality.
Japanese Patent Laid-Open No. 2009-6676 has disclosed a technique which, when the direction in which a nozzle array is arranged has a tilt with respect to the sub scanning direction, acquires the tilt amount based on a test pattern formed by the nozzle array, and corrects a shift of the printing position in accordance with the tilt amount. According to Japanese Patent Laid-Open No. 2009-6676, this correction is performed by changing, in accordance with the above-mentioned tilt amount, the read position of printing data stored in each address of a memory (storage means) in association with each column on a printing medium. Consequently, a dot is printed in an intended column instead of an adjacent column.
As a printing method advantageous in increasing the printing speed or improving the image quality, there is a method in which a printhead has two or more nozzle arrays which discharge ink of the same color, and printing is performed by using these nozzle arrays in order. For example, when the printhead has two nozzle arrays, dots adjacent to each other in the main scanning direction are printed by alternately using the two nozzle arrays. Japanese Patent Laid-Open No. 2009-6676 does not take account of a method of correcting a shift of the printing position in this printing method.